Epidemiological studies have shown a strong relationship between the incidence of suicides, affective episodes and the seasons. In order to understand this association, we have studied patients who regularly become depressed at a certain time of year. Most of these patients become depressed in winter and recover or become hypomanic in spring or summer. We have studied 79 such patients with Seasonal Affective Disorder (SAD). Most are women with an onset of illness in their twenties. During depressions they become lethargic, overeat, oversleep, gain weight and withdraw from friends and family. Although they rarely require hospitalization and hold their jobs, their level of functioning deteriorates. Most Sad patients note that their depressions improve when they travel south in the winter. There is a marked annual rhythm in season of birth of the children of these patients, suggesting that this condition may be a human equivalent of seasonal rhythms of reproduction in animals. There is a strong negative correlation between the timing of depressive symptoms and day length and environmental temperature. A study of bipolar patients revealed that they have a pattern of seasonal vulnerability to depression intermediate between that of SAD patients and normal volunteers. Patients with SAD more frequently have premenstrual mood problems than the general population. Ongoing longitudinal studies involving computer-scanned daily ratings and other measures are aimed at further describing the relationship between climatic variables and mood, sleep and behavior.